Adding my 0.02 cents: I strongly agree with ds store and Kappy as well as other nay-sayers. FileVault 2 (what is shipped with Yosemite and later editions of OS/X) should be avoided unless you are very, very concerned about corporate espionage or some such.
My own experience: I got a brand new Mac Book Pro Retina (2015). I thought I'd try it out and see if there was a noticable performance impact, so I turned on FileVault. After using the machine for a week, these were my findings:
- The encryption of the disk went just fine, but I imagine if something went wrong during the encryption you would be in a lot of trouble (eg., sewercat's unfortunate experience). Make a backup first!
- In general, and as confirmed by several benchmarks that you can find on the web, the performance impact was "not too bad," most of the time taking less than a 10% hit on disk performance.
- However... every now and then the performance impact was huge. For seemingly unrelated, simple operations, I would end up waiting 15 to 20 seconds, including things like:
- Opening the finder on a directory in my Projects/ folder. It took 20+ seconds to show me the 14 folders in my Projects/ folder. For those 20 seconds, the Projects/ folder appeared empty which totally freaked me out because that's where all my work lives.
- Once the Projects/ folder opened, several of the subfolders experienced similar delays. Once the folder went through it's 20+ seconds delay, and was properly loaded, it would behave normally.
- Also rarely, but on a regular basis, access to specific files would seem to "hang" for a period of time. I suspect this was exactly the same problem as I ran into with opening a folder that I hadn't accessed in a while – in this case, the file would simply seem to take forever (well, probably 20+ seconds) to open.
- This kind of problem seemed to repeat, more or less randomly, "here and there." It seemed to be worse after a reboot, so my guess is it was some kind of decryption that would run when I accessed certain information – and for whatever reason, it was horribly, horribly slow.
- During reboot/login, the login time was extended considerably. This is because of how FileVault 2 works: It boots on the rescue partition, and if you successfully login, it then decrypts your entire disk on the fly, and then logs you in (at least, this is how the procedure is described elsewhere). So it's sort of like logging in twice. Not a major problem, and I probably would have kept FileVault 2 turned on if this was the only problem... but the issues under #3 above were totally unacceptable.
- I also noticed that the system would frequently consume a huge amount of processing power. I bought this machine to replace my almost 3-year old Mac Book Pro Retina. I'm well familiar with now often the fans should kick to 100%. The brand new one (with FileVault 2 enabled) would frequently – and I mean as much as half the time it was running – run its fans at 100%. It would even do this will sitting perfectly idle, logged in, doing nothing but sitting there with a couple of programs open. (In the meantime, my older Retina never behaved like this).
After all of this, I wiped the disk, re-installed from scratch (being paranoid), and chose not to use FileVault 2. I recommend that you avoid it. It was nothing but headaches for me.
If you do have some sensitive data that you need to protect, I recommend using utilities like 1Password and Knox. Both work very well, and are superbly integrated with OS/X so you hardly know they are there.
